import sequtils
proc distance*(left, right: string): int = 
  if left.len != right.len:
    raise newException(ValueError, "length should be equal.")

  # for i, x in left:
  #   if x != right[i]:
  #     result.inc

  var matches = zip(left, right)
  keepItIf(matches, it.a != it.b)
  matches.len


